Add support for Jinja2>=3 (to support bookworm)

Need to add support for Jinja3.

Chartkick is causing issues since it's not compatible with jinja2>=3 anymore.

  File "/home/ikus060/workspace/PDSL/rdiffweb.git/.venv/lib/python3.9/site-packages/jinja2/loaders.py", line 563, in load
    return loader.load(environment, name, globals)
  File "/home/ikus060/workspace/PDSL/rdiffweb.git/.venv/lib/python3.9/site-packages/jinja2/loaders.py", line 137, in load
    code = environment.compile(source, name, filename)
  File "/home/ikus060/workspace/PDSL/rdiffweb.git/.venv/lib/python3.9/site-packages/jinja2/environment.py", line 755, in compile
    return self._compile(source, filename)
  File "/home/ikus060/workspace/PDSL/rdiffweb.git/.venv/lib/python3.9/site-packages/jinja2/environment.py", line 693, in _compile
    return compile(source, filename, "exec")  # type: ignore
  File "/home/ikus060/workspace/PDSL/rdiffweb.git/rdiffweb/templates/graphs_times.html", line 39
    _block_vars['prefix'] = l_0_prefix, caller=caller, _block_vars=_block_vars)